The present invention generally relates to communications systems in which a central station selectively communicates with one or more of a plurality of remote stations over communications channels established between the central and remote stations. The invention more particularly relates to mobile telephone systems in which a base station selectively communicates over radio frequency (RF) channels with one or more mobile stations.
Mobile telephone RF systems typically have a terminal at a base station which is capable of establishing a communications path over a selected channel between a calling and called mobile station as well as between mobile stations and called or calling landline parties. Conventionally, signalling transmissions, e.g., tones, are used to establish a communications path between the base and a mobile station following which voice transmissions between the two stations occur. The signalling and voice transmissions between the base station and a mobile station communicating therewith are in a clear, i.e. not secure, mode in that both may be detected and monitored by others by merely tuning in on their shared communications channel.
Various proposals have been suggested for providing secure transmissions, particularly voice transmissions, between a base and mobile station. One such proposal is to encipher transmitted and decipher received signalling and voice transmissions between the base and mobile stations with a common code. In such a system, specialized deciphering equipment would be required to detect and monitor both signalling and voice transmissions between a base and mobile station. However, while this proposal increases privacy of communications, all mobile stations of a mobile telephone system would have the same enciphering/deciphering code associated therewith so that each may receive common tone signalling information, e.g., idle channel tones, emitted by the base station and intended to be received by all mobile stations. As a result, a mobile station of the system would still detect and monitor signalling and voice communications between the base station and a different mobile station. Accordingly, although improved privacy is ensured against outsiders to the system, users of a mobile telephone system employing such techniques are still not guaranteed complete privacy in their communications.
Moreover, it has been discovered that the use of enciphering/deciphering techniques for the signalling portion of the system requires more stringent system design requirements since accurate enciphering and deciphering of transmitted and received signalling, e.g., tone, signals is more difficult than is the case for audio (voice) signals, causing increased signalling errors between a base station and remote station.
Other attempts at providing some measure of security have included furnishing the base and mobile stations with a manually operable switch which permits operators at communicating base and mobile stations to switch their respective units between a clear and secure mode. In the clear mode normal transmissions occur, while transmissions are enciphered in the secure mode. Such a system is described in the paper entitled "A Digitally Encoded Voice Encryption System" by Robert E. Bailey presented at the I.E.E.E. Twenty-Seventh Annual Vehicular Technology Conference, Mar. 16-18, 1977. However, the use of a manually manipulatable clear/secure switch causes problems in that both the base station and mobile station must be set to the same clear or secure mode to ensure that both stations are operating in the same format. If the two stations are not set in the same mode, signalling and voice transmissions will not properly occur.
The present invention has been designed to overcome the above deficiencies in existing and proposed communications systems providing a very private communications system which does not require mannual switching to a secure mode. It has particular applicability to mobile telephone systems, although the principals of the invention are also applicable to any communications system having a central station, and a plurality of remote stations which can communicate with the central station.
One object of the present invention is to provide a simple, reliable, yet highly effective method and apparatus for automatically, or on request, establishing a secure voice transmission channel between a base and mobile station after completion of signalling transmissions between them incident to establishing their shared communications path over a selected channel.
An additional object of the present invention is to provide a method and apparatus for establishing an extremely secure voice transmission channel in a mobile telephone communications system by using a unique code assigned to each mobile station of the system for use by a communicating mobile station at least in deciphering voice transmissions transmitted thereto and the base station in enciphering voice transmissions to the communicating mobile station. The unique code can also be used by the mobile station to encipher voice transmissions to the base station, with the base station using the unique code to decipher an incoming voice transmission from a particular mobile station. Alternatively, all mobile stations may encipher outgoing voice transmissions with a common code, which is also used by the base station to decipher incoming voice transmissions.
An additional object of the present invention is to provide a method and apparatus for establishing a secure voice transmission channel in a mobile telephone system in which the base station after completion of signalling which establishes the communication channel between it and a mobile station, immediately, or in response to a secure service request by a calling or called party switches itself to a secure mode in which voice transmissions to and from the base station are respectively deciphered and enciphered with the code(s) corresponding to that of the mobile station with which it is communicating.
Still another object of the present invention is to provide a method and apparatus for establishing a secure voice transmission path in a mobile telephone system in which each mobile station is responsive to receipt of an enciphered voice transmission from a base station to switch from a clear to a secure mode in which voice transmissions from and to said mobile station are respectively enciphered and deciphered.
These and other objects and advantages of the invention will become clear from the detailed description thereof presented below in connection with the accompanying drawing.